The present invention relates to a hydraulic anti-skid device for a braking system of a vehicle.
A prior art anti-skid device has a construction partially shown in FIG. 3 wherein a spool a which moves rightwards, viewing in this Figure, in response to a pump pressure (actuating pressure) serves as the function of an actuator for a needle valve member b which, when in its closed position, obstructs short-circuit communication between a port c connected to a master cylinder and a port d connected to rear wheel cylinders. When the short-circuit communication is obstructed, the master cylinder pressure is transmitted from the port c to a port e through a one-way check valved passageway which is opened by an expansion piston (not shown in FIG. 3) responsive to the pump pressure and the thus transmitted master cylinder pressure is transmitted from the port e to the port d.
When the rear wheels are about to lock, the pump pressure acting upon the expansion piston is exhausted to cause movement of the expansion piston in the opposite direction. This movement of the expansion piston will permit the check valve to be closed to confine the hydraulic line from the check valve to the rear wheel cylinders and then expand the volume of this hydraulic line. As a result, locking of the rear wheels is prevented or released. In the event of loss of the pump pressure due to, for example, failure of a pump, the needle valve b will be open to permit the short-circuit communication between the port c, communicating with the master cylinder, and port d, communicating with the wheel cylinders, thus establishing an unrestricted fluid passageway from the master cylinder to the rear wheel cylinders thereby to prevent skid preventing operation of the system.
However, with the above-described prior art, a problem is derived from the provision of the needle valve b (FIG. 3) which is arranged for selectively connecting the port d to the port c or port e. This problem will be hereinafter explained. The needle valve b will permit overlap between an opening period of a passage opening f and that of another passage opening h because moving the valve b rightwards will urge a rubber valve member g to open the passage opening h before its tip closes the passage opening f and the subsequent leftward return movement will open the passage opening f before closing the passage opening h.
This overlap will take place if the pump pressure drops and will establish temporarily a passageway permitting the master cylinder pressure to leads from the port c to a pressure reduction chamber, to which the expansion piston projects, through the passage opening f, opening h and port e. If, therefore, the pump pressure drops while the master cylinder is being actuated, the hydraulic fluid from the master cylinder will partially be displaced, through the above-mentioned passageway, to the pressure reduction chamber, tending to urge the expansion piston in such a direction as to expand the volume of the reduction chamber, so that depression degree of the brake pedal will increase considerably under this condition. This depression degree of the brake pedal will increase abruptly to cause the driver to feel uneasy if the above mentioned condition takes place during skid preventing operation when, the pressure in the reduction chamber and thus in the wheel cylinders is at low levels.